Internal combustion engines may be supercharged. Supercharging serves primarily to increase the power of the internal combustion engine. Here, the air required for the combustion process is compressed, as a result of which a greater air mass can be supplied to each cylinder per working cycle. In this way, the fuel mass and therefore the mean pressure can be increased.
Supercharging is a suitable means for increasing the power of an internal combustion engine while maintaining an unchanged swept volume, or for reducing the swept volume while maintaining the same power. In any case, supercharging leads to an increase in volumetric power output and an improved power-to-weight ratio. If the swept volume is reduced, it is thus possible, given the same vehicle boundary conditions, to shift the load collective toward higher loads, at which the specific fuel consumption is lower.
For supercharging, use is often made of an exhaust-gas turbocharger, in which a compressor and a turbine are arranged on the same shaft. The hot exhaust-gas flow is fed to the turbine and expands in the turbine with a release of energy, as a result of which the shaft is set in rotation. The energy supplied by the exhaust-gas flow to the turbine and ultimately to the shaft is used for driving the compressor which is likewise arranged on the shaft. The compressor conveys and compresses the charge air fed to it, as a result of which supercharging of the cylinders is obtained. A charge-air cooler is advantageously provided in the intake system downstream of the compressor, by means of which charge-air cooler the compressed charge air is cooled before it enters the at least two cylinders. The cooler lowers the temperature and thereby increases the density of the charge air, such that the cooler also contributes to improved charging of the cylinders, that is to say to a greater air mass. Compression by cooling takes place.
The advantage of an exhaust-gas turbocharger in relation to a mechanical supercharger is that an exhaust-gas turbocharger utilizes the exhaust-gas energy of the hot exhaust gases, whereas a mechanical supercharger commonly draws the energy required for driving it directly or indirectly from the internal combustion engine, and thus reduces the efficiency. In some cases, a mechanical or kinematic connection is required for the transmission of power between the supercharger and the internal combustion engine.
The advantage of a mechanical supercharger in relation to an exhaust-gas turbocharger is that the mechanical supercharger generates, and makes available, the required charge pressure at all times, specifically regardless of the operating state of the internal combustion engine, in particular regardless of the present rotational speed of the crankshaft. This applies in particular to a mechanical supercharger which can be driven by way of an electric machine.
However, difficulties may be encountered in achieving an increase in power in all engine speed ranges by means of exhaust-gas turbocharging. A relatively severe torque drop is observed in the event of a certain engine speed being undershot. Said torque drop is understandable if one takes into consideration that the charge pressure ratio is dependent on the turbine pressure ratio. If the engine speed is reduced, this leads to a smaller exhaust-gas mass flow and therefore to a lower turbine pressure ratio. Consequently, toward lower engine speeds, the charge pressure ratio likewise decreases. This equates to a torque drop.
One measure to improve the torque characteristic of a supercharged internal combustion engine, for example, is a small design of the turbine cross section and simultaneous provision of an exhaust-gas blow-off facility. Such a turbine is also referred to as a wastegate turbine. If the exhaust-gas mass flow exceeds a critical value, a part of the exhaust-gas flow is, within the course of the so-called exhaust-gas blow-off, conducted via a bypass line past the turbine. This approach has the disadvantage that the supercharging behavior is inadequate at relatively high rotational speeds or in the presence of relatively high exhaust-gas flow rates.
The torque characteristic may also be advantageously influenced by means of multiple exhaust-gas turbochargers connected in series. By connecting two exhaust-gas turbochargers in series, of which one exhaust-gas turbocharger serves as a high-pressure stage and one exhaust-gas turbocharger serves as a low-pressure stage, the compressor characteristic map can advantageously be expanded, specifically both in the direction of smaller compressor flows and also in the direction of larger compressor flows.
The torque characteristic of a supercharged internal combustion engine may furthermore be improved by means of multiple turbochargers arranged in parallel, that is to say by means of multiple turbines of relatively small turbine cross section arranged in parallel, wherein turbines are activated successively with increasing exhaust-gas flow rate.
In the development of internal combustion engines, it is a basic aim to minimize fuel consumption, wherein the emphasis in the efforts being made is on obtaining an improved overall efficiency. Further measures are utilized aside from the supercharging of the internal combustion engine.
Fuel consumption and thus efficiency pose a problem in particular in the case of Otto-cycle engines, that is to say in the case of applied-ignition internal combustion engines. The reason for this lies in the fundamental operating process of the Otto-cycle engine. Load control is generally carried out by means of a throttle flap provided in the intake system. By adjusting the throttle flap, the pressure of the inducted air downstream of the throttle flap can be reduced to a greater or lesser extent. The further the throttle flap is closed, that is to say the more said throttle flap blocks the intake system, the higher the pressure loss of the inducted air across the throttle flap, and the lower the pressure of the inducted air downstream of the throttle flap and upstream of the inlet into the at least two cylinders, that is to say combustion chambers. For a constant combustion chamber volume, it is possible in this way for the air mass, that is to say the quantity, to be set by means of the pressure of the inducted air. This also explains why quantity regulation has proven to be disadvantageous specifically in part-load operation, because low loads demand a high degree of throttling and a large pressure reduction in the intake system, as a result of which the charge exchange losses increase with decreasing load and increasing throttling.
To reduce the described losses, various strategies for dethrottling an Otto-cycle engine have been developed. One approach to a solution for dethrottling the Otto-cycle engine is for example an Otto-cycle engine operating process with direct injection. The direct injection of the fuel is a suitable means for realizing a stratified combustion chamber charge. The direct injection of the fuel into the combustion chamber thus permits quality regulation in the Otto-cycle engine, within certain limits. The mixture formation takes place by the direct injection of the fuel into the cylinders or into the air situated in the cylinders, and not by external mixture formation, in which the fuel is introduced into the inducted air in the intake system.
Another option for optimizing the combustion process of an Otto-cycle engine includes an at least partially variable valve drive. By contrast to conventional valve drives, in which both the lift of the valves and the control timing are invariable, these parameters which have an influence on the combustion process, and thus on fuel consumption, can be varied to a greater or lesser extent by means of variable valve drives. If the closing time of the inlet valve and the inlet valve lift can be varied, this alone makes throttling-free and thus loss-free load control possible. The mixture mass or charge air mass which flows into the combustion chamber during the intake process is then controlled not by means of a throttle flap but rather by means of the inlet valve lift and the opening duration of the inlet valve. Variable valve drives are however very expensive and are therefore often unsuitable for series production.
A further approach to a solution for de-throttling an Otto-cycle engine is offered by cylinder deactivation, that is to say the deactivation of individual cylinders in certain load ranges. The efficiency of the Otto-cycle engine in part-load operation can be improved, that is to say increased, by means of such partial deactivation because the deactivation of one cylinder of a multi-cylinder internal combustion engine increases the load on the other cylinders, which remain operational, if the engine power remains constant, such that the throttle flap may be opened further in order to introduce a greater air mass into said cylinders, whereby de-throttling of the internal combustion engine is attained overall. During the partial deactivation, the cylinders which are permanently operational operate in the region of higher loads, at which the specific fuel consumption is lower. The load collective is shifted toward higher loads.
The cylinders which remain operational during the partial deactivation furthermore exhibit improved mixture formation owing to the greater air mass or mixture mass supplied.
Further advantages with regard to efficiency are attained in that a deactivated cylinder, owing to the absence of combustion, does not generate any wall heat losses owing to heat transfer from the combustion gases to the combustion chamber walls.
Even though diesel engines, that is to say auto-ignition internal combustion engines, owing to the quality regulation on which they are based, exhibit greater efficiency, that is to say lower fuel consumption, than Otto-cycle engines in which the load—as described above—is adjusted by means of throttling or quantity regulation with regard to the charge of the cylinders, there is, even in the case of diesel engines, potential for improvement and a demand for improvement with regard to fuel consumption and efficiency.
One concept for reducing fuel consumption, also in the case of diesel engines, is cylinder deactivation, that is to say the deactivation of individual cylinders in certain load ranges. The efficiency of the diesel engine in part-load operation can be improved, that is to say increased, by means of a partial deactivation, because, even in the case of the diesel engine, in the case of constant engine power the deactivation of at least one cylinder of a multi-cylinder internal combustion engine increases the load on the other cylinders that are still operational, such that said cylinders operate in regions of higher loads, in which the specific fuel consumption is lower. The load collective in part-load operation of the diesel engine is shifted toward higher loads.
With regard to the wall heat losses, the same advantages are attained as in the case of the Otto-cycle engine, for which reason reference is made to the corresponding statements given.
In the case of diesel engines, the partial deactivation is also intended to prevent the fuel-air mixture from becoming too lean in the context of the quality regulation in the presence of decreasing load as a result of a reduction of the fuel quantity used.
However, the inventors herein have recognized that multi-cylinder internal combustion engines with partial deactivation and the associated methods for operating said internal combustion engines suffer from various issues, as will be explained briefly below.
If, for the purpose of the partial deactivation, the fuel supply to the deactivatable cylinders is stopped, that is to say discontinued, the deactivated cylinders continue to participate in the charge exchange if the associated valve drive of said cylinders is not deactivated or cannot be deactivated. The charge exchange losses thus generated by the deactivated cylinders lessen, and counteract, the improvements achieved with regard to fuel consumption and efficiency by means of the partial deactivation, such that the benefit of the partial deactivation is at least partially lost, that is to say the partial deactivation in fact yields an altogether less pronounced improvement.
In practice, it is not always expedient for the above-described disadvantageous effects to be remedied through the provision of switchable valve drives, because switchable valve drives such as variable valve drives are very expensive and exhibit only limited suitability for series production.
Furthermore, in the case of internal combustion engines supercharged by exhaust-gas turbocharging, switchable valve drives would lead to further problems because the turbine of an exhaust-gas turbocharger is configured for a certain exhaust-gas flow rate, and thus generally also for a certain number of cylinders. If the valve drive of a deactivated cylinder is deactivated, the overall mass flow through the cylinders of the internal combustion engine is initially reduced owing to the omission of the mass flow through the deactivated cylinders. The exhaust-gas mass flow conducted through the turbine decreases, and the turbine pressure ratio generally also decreases as a result. This has the effect that the charge pressure ratio likewise decreases, that is to say the charge pressure falls, and only a small amount of fresh air or charge air is or can be supplied to the cylinders that remain operational. The small charge-air flow may also have the effect that the compressor operates beyond the surge limit.
It would however in fact be necessary for the charge pressure to be increased in order to supply more charge air to the cylinders that remain operational, because in the event of deactivation of at least one cylinder of a multi-cylinder internal combustion engine, the load on the other cylinders, which remain operational, increases, for which reason a greater amount of charge air and a greater amount of fuel is supplied to said cylinders. The drive power available at the compressor for generating an adequately high charge pressure is dependent on the exhaust-gas enthalpy of the hot exhaust gases, which is determined significantly by the exhaust-gas pressure and the exhaust-gas temperature, and the exhaust-gas mass or the exhaust-gas flow.
In the case of Otto-cycle engines, by opening the throttle flap, the charge pressure can be easily increased in the load range relevant for partial deactivation. This possibility does not exist in the case of the diesel engine. The small charge-air flow may have the effect that the compressor operates beyond the surge limit.
The effects described above lead to a restriction of the practicability of the partial deactivation, specifically to a restriction of the engine speed range and of the load range in which the partial deactivation can be used. In the case of low charge-air flow rates, it is not possible, owing to inadequate compressor power or turbine power, for the charge pressure to be increased in accordance with demand.
The charge pressure during partial deactivation, and thus the charge-air flow rate supplied to the cylinders that remain operational, could for example be increased by a small configuration of the turbine cross section and by simultaneous exhaust-gas blow-off, whereby the load range relevant for a partial deactivation would also be expanded again. This approach however has the disadvantage that the supercharging behavior is inadequate when all the cylinders are operated.
The charge pressure during partial deactivation, and thus the charge-air flow rate supplied to the cylinders that are still operational, could also be increased by virtue of the turbine being equipped with a variable turbine geometry, which permits an adaptation of the effective turbine cross section to the present exhaust-gas mass flow. The exhaust-gas back pressure in the exhaust-gas discharge system upstream of the turbine would then however simultaneously increase, leading in turn to higher charge-exchange losses in the cylinders that are still operational.
Thus, the inventors herein provide a system to at least partly address the above issues. In one example, a system includes a supercharged internal combustion engine having at least two cylinders arranged into a first group and a second group, each cylinder having at least one outlet opening adjoined by a respective exhaust line for discharging exhaust gases via an exhaust-gas discharge system and at least one inlet opening adjoined by a respective intake line for supply of charge air via an intake system. Each cylinder of the first group is configured to be operational even during partial deactivation of the internal combustion engine, and each cylinder of the second group is configured to be a load-dependently switchable cylinder. Each exhaust line of each cylinder of the first group merges to form a first overall exhaust line, thus forming a first exhaust manifold, and each exhaust line of each cylinder of the second group merges to form a second overall exhaust line, thus forming a second exhaust manifold. Each intake line of each cylinder of the first group merges to form a first overall intake line, thus forming a first intake manifold, and each intake line of each cylinder of the second group merging to form a second overall intake line, thus forming a second intake manifold. The system further includes a first exhaust-gas turbocharger having a first turbine arranged in the exhaust-gas discharge system and a first compressor arranged in the intake system, the first turbine and first compressor being arranged in on a first rotatable shaft, and a second exhaust-gas turbocharger having a second turbine arranged in the exhaust-gas discharge system and a second compressor arranged in the intake system, the second turbine and second compressor being arranged in on a second rotatable shaft. The first turbine is arranged in the first overall exhaust line of the first cylinder group and the second turbine arranged in the second overall exhaust line of the second cylinder group, and the first compressor is arranged in the first overall intake line of the first cylinder group and the second compressor arranged in the second overall intake line of the second cylinder group, the first and second compressor arranged in parallel. The first intake manifold of the first cylinder group and the second intake manifold of the second cylinder group are connectable to one another via a connection, a first shut-off element being arranged in the connection.
The internal combustion engine according to the disclosure is equipped with at least two exhaust-gas turbochargers and consequently with more than one turbine in the exhaust-gas discharge system. The turbines of the at least two exhaust-gas turbochargers are arranged in parallel in the exhaust-gas discharge system, with each cylinder group being assigned a turbine. This yields a considerable improvement in supercharging behavior, that is to say in the torque characteristic of the internal combustion engine, in particular during partial deactivation.
Each turbine may be configured for the exhaust-gas flow rate of the associated cylinder group, that is to say for the number of cylinders of the respective group. Then, if the deactivatable cylinder(s) of the second group is deactivated, this no longer necessarily has an influence on the exhaust-gas flow rate conducted through the turbine of the first group, as a result of which the turbine pressure ratio of said first turbine does not necessarily decrease. The charge pressure ratio does not decrease, and sufficient charge air is supplied to the cylinders that remain operational.
According to the disclosure, it is specifically the case that the compressors of the at least two exhaust-gas turbochargers are likewise arranged in parallel in the intake system, with each cylinder group being assigned a compressor. The compressor of the first exhaust-gas turbocharger is arranged in the first overall intake line of the first cylinder group, and is thus assigned to the first cylinder group. The compressor of the second exhaust-gas turbocharger is arranged in the second overall intake line of the second cylinder group, and is thus assigned to the second cylinder group. Consequently, the compressor of the second exhaust-gas turbocharger (also referred to as the second compressor), may be deactivated, for example separated from the rest of the intake system by way of a shut-off element, during the partial deactivation owing to the absence of demand. The associated second turbine is in any case not supplied with exhaust gas. The supply of charge air to the deactivated cylinders is preferably stopped.
According to the disclosure, the intake systems of the cylinder groups are connectable via a connection, the first intake manifold of the first cylinder group and the second intake manifold of the second cylinder group being connectable to one another via the connection, and a first shut-off element being arranged in the connection. In the context of the present disclosure, the intake manifold comprises in each case the intake lines of the associated cylinder group, that part of the associated overall intake line which leads as far as the compressor arranged in the overall intake line, and in some cases a plenum provided in between.
The physical feature whereby the first intake manifold and the second intake manifold are connectable to one another via the connection, but can be separated from one another by virtue of the first shut-off element arranged in the connection being closed, opens up numerous advantageous possibilities for the operation of the internal combustion engine.
During the partial deactivation, in the case of which the at least one switchable cylinder of the second group is deactivated, the first shut-off element may be closed. Then, the compressor of the first exhaust-gas turbocharger (also referred to as the first compressor), supplies charge air only to those cylinders of the first group which are operational even during partial deactivation, with the intake system of the second cylinder group being separated from the intake system of the first cylinder group. Then, the first compressor does not deliver charge air to the deactivated cylinders of the second group, nor does it deliver charge air into the compressor of the second exhaust-gas turbocharger, which is preferably deactivated during partial deactivation. Both would be disadvantageous, and therefore also undesirable.
With increasing load, it is then firstly possible for the first shut-off element to be opened, and the deactivated cylinders of the second group may be activated, such that the first compressor supplies charge air to all of the cylinders before the compressor of the second exhaust-gas turbocharger is activated in order to ensure or assist the provision of demanded charge pressure in the presence of further increasing load.
This approach also has the advantage that the second turbine assigned to the second cylinder group is accelerated again already before the compressor of the second exhaust-gas turbocharger is activated.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.